JPH0722678B2 - Method for removing SOx from flue gas and other gas streams using absorbents - Google Patents

Abstract

A process of removing noxious sulfur oxides from gas streams using heated layered double hydroxide (LDH) sorbents is described. The sorbent compositions contain metal components incorporated into the galleries of the LDH structures in the form of metal-containing oxo-anions, to promote the oxidation of sulfur dioxide.

Description

DETAILED DESCRIPTION OF THE INVENTION REFERENCE RELATED APPLICATION This application is based on US patent application Se filed January 18, 1990.
It is a CIP with rial No. 07 / 466,984.

BACKGROUND OF THE INVENTION (1) Technical Field The present invention relates to a method for removing sulfur dioxide and sulfur trioxide from a gas mixture using an absorbent. In particular, the present invention is
It relates to the use of crystalline layered double hydroxides with an intermediate layer of ions that oxidize sulfur dioxide to sulfur trioxide.

(2) Conventional technology In a fossil fuel-fired power plant, sulfur contained in the supplied coal is oxidized into sulfur oxides (SO ₂ and SO ₃ , commonly referred to as “SOx”) during combustion, Is released into the atmosphere through the chimney, causing it to attach as "acid rain". Analysis of flue gas from a coal-fired power plant before desulfurization revealed 0.5% to 0.2% SO ₂ and approximately
0.005% SO ₃ is shown. US Environmental Protection Agency (EPA)
Has ordered the control of SOx emissions, and various studies have been conducted to develop methods for its removal from flue gas streams.

The formation of SOx during the combustion process can be reduced by improving the burner structure and combustion system, changing operating conditions, and using fuels with low sulfur content. The most common and cheap way to reduce SOx emissions is to add a reactive dry absorbent to the fuel. Therefore, SOx removal is currently mostly achieved using lime (CaO) or limestone (CaCO ₃ ). Some other basic absorbents such as MgO, ZnO have also been found to be effective in removing SOx.
For an overview of dry absorbents, see, for example, Compa (Ko
mppa) V. "Cyclic Absorption Method for Removal of SOx and NOx in Flue Gas-Review" (Dry Adsorption Process for
Removal of SOx and NOx in Flue Gases−a revie
w), Paperii ja Puu, 5 , 401-405 (1986).

The use of Group 2 (formally Group IIA) metal oxides such as magnesium and calcium oxides as SOx absorbents is described in several patent documents, and in recent examples U.S. Pat. No. 3,835,031. And No. 3,699,037. Some other metal oxides having various effects as SOx absorbents are described in U.S. Pat.No. 4,153,534, which include sodium, scandium, titanium, iron, chromium, molybdenum, manganese, cobalt, Contains oxides such as nickel, copper, zinc, cadmium, rare earth metals, and lead.

In a typical coal-fired power plant, a crushed absorbent, such as lime or limestone, is added with the coal into the boiler or sprayed into the tower as a slurry to contact the flue gas. SO ₂ reacts with calcium hydroxide to form a calcium sulfite slurry, which is then partially oxidized with air to calcium sulfate. In this way the sulfur oxides are retained as innocuous solid compounds, which can be removed from the flue gas by electrostatic precipitation or other standard methods. Such a method is potentially attractive for retrofitting existing power plants. Because no major structural changes are necessary.

The main problem with this type of method is the low availability of oxide absorbents. The SOx absorption rate decreases rapidly with increasing conversion due to the mass transfer limit and the low reactivity of SO ₂ . Therefore, if the available contact time is relatively short, only a small portion of the absorbent will react. In principle, the problem of low availability of the absorbent could be solved by reducing the particle size, but in practice the particle size required for reasonable utilization is too small and is It will not be economically achievable with the crushing method.

Thermodynamic calculations indicate that the capture of sulfur trioxide by metal oxides is more convenient than sulfur dioxide. Some experimental results suggest that catalytic oxidation of sulfur dioxide to sulfur trioxide favors flue gas desulfurization. Kocaefe and Karman, Cand. J. Chem. Eng., 63 , 971-977 (198
In 5), the reaction rate of Ca, Mg and ZnO with SO ₃ is
It shows that the reaction rate of the same oxide and sulfur dioxide is larger under the same conditions. Further, by including Fe ₂ O ₃ (as a SO ₂ oxidation catalyst), lime can be used more effectively. With a small amount of Fe ₂ O ₃ added, a higher initial uptake rate and a much higher final lime conversion (80 ~
90%) give both. In the absence of an oxidation catalyst, SO ₂
The absorption rate drops sharply at a conversion of about 70%.

Similar methods have been used to design SOx absorbents for the fluid catalytic cracking (FCC) treatment of petroleum. Among other things, these absorbents are mostly alkaline earth metal spinels containing one or more other metal components capable of oxidizing sulfur dioxide. For example, US Pat. Nos. 4,472,532 and 4,492,678 relate to formulating iron, chromium, vanadium, manganese, gallium, boron, cobalt, platinum, and cerium as oxidation catalysts.

Thus, when designing an improved absorbent to remove SOx, (i) oxidize SO ₂ to SO ₃ , (ii) chemisorb the formed SO ₃ , and (iii) regenerate the absorbent. In order to be able to liberate the absorbed SOx, or to form a stable substance which can completely dispose of the used solid absorbent, substances must be synthesized. The SOx released from these used absorbents is safely captured and can be used to produce sulfuric acid or sulfur.

Recently European patent application EP-A 278 535 describes catalyst compositions suitable for refining petroleum feeds rich in sulfur and metals. That is, the catalyst composition according to the disclosure discloses a zeolitic material such as ZSM-5, ZSM-11, etc. having catalytic activity for hydrocarbon conversion, LDH for binding and removal of sulfur oxides.
It includes a structured anionic clay material and a matrix material such as kaolin or alumina. The preferred catalyst composition comprises an anionic clay composition in an amount of 1 to 30%, based on the total catalyst composition.

There is a need for absorbent compositions suitable for reducing SOx from flue gas streams, especially flue gas streams from lime power plants. C due to the low reactivity of SO ₂ and the mass transfer limit
In order to overcome the low availability of common oxide absorbents such as aO and MgO, there is a need to develop absorbent compositions that can better uptake SOx within a short period of time.

LDH is a group of anionic clay minerals. They have multiple layers of positively charged metal hydroxide with anions and some water molecules between them. Most common LDHs are based on multiple oxides of the main metal groups such as Mg and Al and transition metals such as Ni, Co, Cr, Zn and Fe. These clays have a structure similar to brucite [Mg (OH) ₂ ] in which magnesium ions are surrounded by hydroxyl groups in an octahedral shape and the resulting octahedrons share edges to form an infinite sheet. Have. Such LDH
Then some of the magnesium is a trivalent ion, for example Al
^It is isomorphically replaced by ³⁺ . In that case, Mg ²⁺ ,
The Al ³⁺ , OH ⁻ layers are positively charged, and it is necessary to balance the charges by inserting anions between the layers.

One such anionic clay is hydrotalcide, in which carbonate ions are intervening anions, the formula:
It has an ideal unit cell of [Mg ₆ Al ₂ (OH) ₁₆ ] (CO ₃ ) .4H ₂ O. However, the Mg / Al ratio in hydrotalcite-like materials varies between 1.7 and 4, and various other divalent and trivalent ions may be substituted for Mg and Al. Further, the anion is a carbonate ion in ^{_{hydrotalcite, NO 3 -, Cl -,}} OH -, can be varied in synthesis by such a number of simple anions of SO ₄ ^2-like. Based on their structure, these LDHs belong to the Pyroaurite-Sjogrenite group, and the true positively charged brissite-like layers form a lattice of carbonic acid groups and oxygen atoms of water molecules. It alternates with the layers distributed on the dots.

Hydrocalumite and related synthetic compounds also have a layered structure with alternating positively charged metal hydroxide layers and intermediate layers containing anions and water. The hydroxide layer contains a particular combination of metal ions derived from divalent calcium cations on the one hand and iron, especially trivalent metal cations such as aluminum. The intermediate layer is OH ^-, S
It contains anions such as O ₄ ²⁻ , Cl ⁻ , NO ₃ ⁻ , especially CO ₃ ²⁻ . The general formula for that group is [Ca ₂ M ³⁺ (OH) ₆ ] X · yH
₂ O (where M ³⁺ is a trivalent cation, typically Al
³⁺ , X is a monovalent anion or the same amount of a higher charged anion, and y is 2-6). As with the Pyroaurite-Shogrenite group, the main layers alternate with the intermediate layers, the main layers have the composition [Ca ₂ M ³⁺ (OH) ₆ ] ⁺ , and the intermediate layers have water molecules. And an anion X. However, due to the size difference between Ca ²⁺ and Al ³⁺ ions, the M ²⁺ : M ³⁺ ratio is fixed at 2: 1 and their arrangement is well organized. A known natural mineral in the group is hydrocalumite, whose composition is approximately [Ca ₂ Al (O
H) ₆ ] (OH) _0.75 (CO ₃ ) _0.125・ 2.5H ₂ O, but [C
a ₂ Fe (OH) ₆ ] (SO ₄ ) _0.5 / 3H ₂ O, [Ca ₂ Al (OH) ₆ ]
There are many synthetic congeners such as (OH) 6H ₂ O.

LDH synthesis is generally straightforward, the so-called "precipitation method" being the most common. If a carbonate-containing product is desired, an aqueous solution of magnesium and aluminum salts, i.e. nitrates or chlorides, is converted to an aqueous solution of sodium hydroxide / carbonate at room temperature with good stirring. The amorphous precipitate obtained is then heated at 60 to 200 ° C. for several hours to obtain a crystalline substance. The synthesis is completed in quantitative yield by washing and drying. By using this precipitation method, all or part of Mg ²⁺ is replaced by other M ^II ions such as Ca ²⁺ , Zn ²⁺ , Cu ²⁺ , or Al ³⁺ is replaced by Fe ³⁺ , Cr ³ Other such as ⁺ etc.
It can also be replaced with M ^III ions.

An important feature in the synthesis of these materials is the variation in the nature of the intervening anions. The production of hydrotalcite-like materials with anions other than carbonate in pure form requires special procedures. This is because LDH preferentially takes up carbonate ions over other anions. Most of the time, a relatively small anion is introduced into the LDH structure by the precipitation method using the desired anion solution instead of carbonate. However, in these methods, the synthesis must be carried out in airless conditions to prevent carbon dioxide pollution by carbon dioxide in the atmosphere. Methods for producing these LDHs are described in prior art publications, and the following review journal articles are of particular reference. Solid State by SL Suib and others
Ionics, 26 , 77-86 (1988), and WT Reichel (Reic
Chemtech, 58-63 (1986).

Methods of synthesizing hydrotalcite-like clays have also been the subject of many patents. U.S. Patent No. 3,796,79 by Miyata et al.
Nos. 2, 3,879,523 and 3,879,525 describe cation layers and anions containing small transition metal anions such as CrO ₄ ²⁻ , MoO ₄ ²⁻ , and Mo ₂ O ₇ ²⁻ . Hydrotalcite-like derivatives with both substitutions are described. Both compositions and methods of manufacture are described, said compositions being said to be useful for catalytic purposes, absorbents, desiccants and the like. Synthetic hydrotalcite-like derivatives with small anions including transition element anions and large organic anions such as long-chain aliphatic dicarboxylates have been shown to catalyze aldol condensation effectively Has been done.

LD large anions such as transition metal polyoxoanions
It is not easy to introduce it between H layers. This is L
Ion exchange method is required following DH synthesis. Pinabea and Kwon are J. Am. Chem. Soc., 110 , 3653 (1988).
And some polyoxometallates containing V ₁₀ O ₂₈ ^6-
A practical example of pillaring into a hydrotalcite structure containing Zn and Al metal ions in the layer is shown. U.S. Patent No. 4, by Woltermann,
No. 452,244 describes the preparation of several polyoxometallate LDHs. However, no XRD or analytical data has been given to prove the purity of these substances. Recently, US Pat. No. 4,774,212 by Drezdon discloses the preparation of some Mg / Al hydrotalcite-like materials containing transition metal polyoxoanions.

The pyrolytic properties of LDH, especially hydrotalcite-like materials, have been studied in detail. For example, the weight of hydrotalcite [Mg ₆ Al ₂ (OH) ₁₆ ] (CO ₃ ) ・ 4H ₂ O is reduced in two stages by thermal decomposition. First, it loses four intervening water molecules when heated to 200 ° C, while maintaining the skeletal hydroxide and interlayer carbonate ions. Further heating to 275 ° C-450 ° C results in the simultaneous loss of hydroxyl and carbonate ions as water and carbon dioxide, respectively. These magnesium-aluminum solid solutions have a sodium chloride type structure having a cation defect. Raihiru is J.Catal. 101, at 352-359 (1986),
This heating of hydrotalcite is about 120 to about 230 m ² / g
It is shown to be accompanied by an increase in the specific surface area to (N ₂ , BET) and a doubling of the pore volume (0.6 to 1.0 cm ³ / g, Hg penetration method). When these solid solutions are further heated to higher temperatures,
Like reactivity, it causes a decrease in specific surface area. MgO at 1000 ° C
And the formation of spinel phase, MgAl ₂ O ₄ , is observed.

OBJECTIVES It is therefore an object of the present invention to provide a novel absorbent composition which can oxidize SO ₂ to SO ₃ , remove SO ₃ and then regenerate and reuse. These and other objects will become increasingly apparent with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a layered double hydroxide (LDH) [Mg ₆ Al ₂ (O
FIG. 3 is a graph plotting thermogravimetric analysis (TGA) of SO ₂ incorporated by (H) ₁₆ ] (FeO ₄ ) .xH ₂ O.

FIG. 2 is a graph showing the temperature dependence of SO ₂ uptake by [Mg ₆ Al ₂ (OH) ₁₆ ] (V ₁₀ O ₂₈ ) _1/3 · xH ₂ O.

Figure 3 shows [Zn ₂ Al (OH) ₆ ] (SiV ₃ W ₉ O ₄₀ ) _1/7 · xH ₂ O
3 is a graph plotting thermal decomposition of TGA and used absorbent for SO ₂ taken in by.

FIG. 4 shows different Mg-containing absorbents produced according to the present invention and those currently used in flue gas desulfurization.
6 is a graph comparing SO ₂ uptake rates.

General Description The present invention provides a method for removing SOx components from flue gas and other gas streams by contacting a heated absorbent composition with the gas stream, wherein the absorbent has the formula: [M _{^{1- x II Mx III (OH)}} 2] (an -) x / n · yH 2 O ( wherein, M ^II is a divalent metal cation, M ^III is to form a metal oxide, SO Is a trivalent metal cation selected from the group consisting of metal cations capable of reacting with ₂ to form metal sulfites and with SO ₃ to form metal sulfates, wherein A is The layered double hydroxide structure contains at least one metal atom that provides the oxidation of sulfur dioxide to sulfur trioxide in an amount sufficient to promote the oxidation of sulfur dioxide to sulfur trioxide under sulfur dioxide oxidation conditions. Charge n-
Is an intermediate layer anion, and x is 0.8 to 0.12), and the SOx removal method is selected from the group consisting of the crystalline layered double hydroxide structure and the heat-treated derivative of the double hydroxide thereof.

Highly basic characteristics (pKa ≦ 35) due to thermal decomposition of LDH
And an active metal oxide having a large specific surface area is formed. These heat treated materials should have unusually well dispersed reactive metal centers, judging by their catalytic properties. Due to these properties we heat treated LD
H materials have been synthesized and used as absorbents suitable for flue gas desulfurization. SO ₂ oxidation catalysts (usually transition metals) and SO ₃ absorbents (usually metal oxides from Group IA or IIA) can be easily incorporated into or between layers of LDH materials using inexpensive starting materials. Can be incorporated.

The present invention therefore relates to the use of layered double hydroxide compositions, in particular hydrotalcite-like and hydrocalumite-like substances, for absorbing SOx from flue gas streams. It also describes the incorporation of other metal components, preferably transition metal ions, which are capable of promoting the oxidation of sulfur dioxide to sulfur trioxide at the calcination temperature. These second metal components are incorporated into the LDH by intercalation in the form of metal polyoxoanions.

LDH sorbent reacts with SO ₂ at various temperatures, in particular 500 to 1000, they absorbers are particularly used to reduce the emission of sulfur oxides from coal combustion boiler.

Here, we also consider a method of recycling spent absorbents by (i) removing the entrapped SOx at high temperature and / or (ii) discarding them as solid waste.

Accordingly, the present invention provides the formula: [M _1- x ^II Mx ^III (OH) ₂ ] (A _{x / n} ) n ⁻ · yH ₂ O wherein M ^II is a divalent metal and M ^III is A is a trivalent metal, A is an anion having a charge of n, and x is 0.8 to 0.1.
2) The layered double hydroxide composition suitable for absorbing SOx, especially hydrotalcite-like substances and hydrocalumite-like substances are used. Both M ^II and / or M ^III are wholly or partially metallic (preferably first) which forms a reactive basic oxide capable of reacting with SOx at the calcination temperature (preferably above 500 ° C.). II A, II B, and III A). Therefore, the preferred LDH for use in the present invention comprises these metals, especially magnesium and aluminum, in a brucite-like [Mg (OH) ₂ -like] layer. Other alkaline earth metal ions such as calcium, strontium, barium, and mixtures thereof may replace all or part of the magnesium ion.

In a broad sense, the present invention contemplates the use of these LDH sorbents to control sulfur oxides from gas streams, especially gas streams from coal burning boiler units. These devices include boilers, economizers, and dust collectors such as electrostatic precipitators or bag filter rooms. It is taken into account in the present invention that these, especially the boilers (700 to 1000 ° C.) are charged with the absorbent together with the coal or that they are placed in an electrostatic precipitator (high temperature 400 to 500 ° C.). There is. For example, the LDH absorbent is heat treated in a temperature programmed thermogravimetric analysis balance in a stream of air or nitrogen at a temperature in the range of 500-1000 ° C. and SOx gas is introduced. The amount of SO ₂ that reacted with the absorbent was detected as a weight gain.

Hydrotalcite absorbent, [Mg ₆ Al ₂ (OH) ₁₆ ] (C
The reaction of O ₃ ) .xH ₂ O (abbreviated as Mg ₃ Al-LDH) with SO ₂ will give a general description of typical experimental methods used to study LDH reactivity. The hydrotalcite was heated to 700 ° C in a stream of air in a thermogravimetric analysis thermostat controlled at a rate of 5 ° C / min. The sample was heated at 700 ° C. for an additional hour. During the calcination process, the sample lost weight due to the removal of CO ₂ and H ₂ O. This sample next
It was exposed to a gas stream containing SO ₂ at a flow rate of 200 ml / min at a concentration of 0.5% by volume. A weight gain of 6.2% was observed. This corresponded to the amount of SOx absorbed that formed the metal sulfate, MgSO ₄ . The diffraction peak observed in the X-ray diffraction pattern of the final product was crystalline MgSO _4.
, Indicating that the magnesium point had become a reactive material at this temperature. The weight gain observed corresponded to a 4.4% conversion of MgO to MgSO ₄ . However, this value is low compared to other modified sorbents described later in this invention.

LD to accelerate the oxidation of sulfur dioxide to sulfur trioxide.
A third metal component was incorporated into H. The third metal component is preferably a transition metal, a rare earth metal, or a metal component selected from Group IVA of the periodic table. Some of the known transition metal and transition metal oxide catalysts suitable for SO ₂ oxidation include Pt, W
O ₃ , Ag, Ag ₃ VO ₄ , Cu ₃ (VO ₄ ) ₂ , V ₂ O ₅ , Fe ₂ O ₃ , TiO ₂ , C
uO, CrO ₃ , MnO ₂ , PbO ₂ , MoO ₃ , CeO ₂ , Cr ₂ O ₃ , SnO ₂ and ZnO are included. Platinum is an excellent oxidation catalyst, and vanadium pentoxide, and other oxides such as iron oxides, are also particularly effective in catalyzing the oxidation of SO ₂ to SO ₃ . For example, Z. by Neuwmann et al.
See Electrochem. 38 , .304-310 (1932). Catalysis for these oxides will include the following steps: absorption of SO ₂ to form sulfite, oxidation of sulfite to sulfate, and sulfate with evolution of SO _3. Disassembly of. Therefore, for certain metal oxide absorbents, SO ₂
It is very important to choose a good metal oxide catalyst for oxidation. The conditions for a good catalyst can be comparable to those for a SO ₂ absorbent. All three steps for the catalyst are surface reactions and will occur at the reaction temperature. In the case of SO ₂ sorbents, the first two steps will occur as the main reaction converting most of the sorbent to sulfate during absorption at the reaction temperature. The last step will occur at higher temperatures.

Particularly good results have been obtained when a transition metal, in particular iron or vanadium, is introduced into the LDH as a third metal, as disclosed in the present invention. These metals have been incorporated into LDH absorbent compositions by structural means during synthesis. As disclosed in the present invention, the metal components are LDH structures,
Replaced by a portion or anion whole, containing sulfur dioxide oxidation metal described before _{^{- [M 1- x II Mx III}} (OH) 2] (A x / n) n - of · H ₂ O An In a similar manner between multiple [M _1- x ^II Mx ^III (OH) ₂ ] layers (galler
Introduced in y). Therefore, copper, zinc, cobalt, iron,
Cadmium, mercury, lead, manganese, tin, nickel, palladium, chromium, vanadium, manganese, gallium, boron, cobalt, and by anion containing such metal ions mixtures thereof, an interlayer anion of LDH structure in An ^-
May be replaced in whole or in part.

Therefore, those anions are oxalate (ox), Fe (o
x) ₃ ³⁻ , simple oxo anions, eg CrO ₄ ²⁻ , FeO ₄
^2-, MnO ₄ ^-, or Oki greater oxo anion having a Do charge, for ^{_{example, V 10 O 28 6-, W}} 7 O 24 6-, Mo 7 O 24 6- , etc.,
Or BVW ₁₀ O ₄₀ ^7- , H ₂ W ₁₂ O ₄₀ ^6- , SiV ₃ with Keggin type structure
C ₉ O ₄₀ ⁷ -Polyoxometallate or vacancy type (defect)
BCoW ₁₁ O ₃₉ ^7- , SiW ₁₁ O ₃₉ ^8- , PMo ₂ W ₉ O ₃₉ with Keggin structure
Anions such as ⁷ -polyoxometallate, or BCoW ₁₂
It can be one or more of metal anion complexes such as anions such as O ₄₂ ^8- containing fused Keggin-type structures. Inserting these anions of different sizes not only helps to oxidize SO ₂ to SO ₃ , but also LDH
Reactive SO that also helps to create fine pores in the structure
Make the _two easily accessible.

Introduction of by-product anions into the hydrotalcite structure
It was carried out by substituting CO ₃ ²⁻ from between the layers as follows: A hydrotalcite material with suitable Mg / Al ratio was first calcined at 500 ° C. and then hydrolyzed in aqueous solution at airless condition. decompose as the interlayer anion OH ^- reshaping hydrotalcite-like LDH containing. OH in the inter-layer ^- it is, FeO ₄
^2- , CrO ₄ ^2- , Fe (ox) ₃ ^3-, etc., or larger anions such as V ₁₀ O ₂₈ ^6- , W ₇ O ₂₄ ^6- , H ₂ W ₁₂ O ₄₀ ^6- , BVW ₁₀ O ₄₀ ^7- , Si
Much larger anions with a Keggin type structure such as V ₃ W ₉ O ₄₀ ^7-, various anions such as vacancy type structures such as SiW ₁₁ O ₃₉ ^9- , BCoW ₁₁ O ₃₉ ^7-, etc. Can be easily replaced. The separated product showed X-ray diffraction peaks corresponding to crystalline phases with a clear basal plane spacing (Table 1).

The introduction of the by-product polyoxomerate anion into Zn / Al-LDH was carried out in the same manner as [Zn ₂ Al (OH) ₆ ] (NO ₃ ) xH.
Carried out starting from a hot aqueous suspension of ₂ O (referred to as Zn ₂ Al-LDH), α- [H ₂ W ₁₂ O ₄₀ ] ^6- and α- [SiV ₃ W ₉
O ₄₀ ] ^7- α-Keggin type ion, or SiW ₁₁ O ₃₉ ^9- , BCoW ₁₁ O
It was found to undergo a complete intercalation ion exchange reaction with an aqueous solution of a polyoxometallate anion containing vacancy (defect) Keggin-type ions such as ₃₉ ^7- . Anions having a low charge, such as [PW ₁₂ O ₄₀ ] ^3- and [SiW ₁₂ O ₄₀ ] ^4- do not show ion exchange, whereas intermediate anions [eg
[PCuW ₁₁ O ₃₉ (H ₂ O)] ^5- ] shows partial insertion. Furthermore,
Polyoxometallate anions having a β-Keggin structure such as β- [SiV ₃ W ₉ O ₄₀ ] ^7- undergo partial insertion. "Pillaring of a Layered Double Hydroxide by P." by Kuon T. and Pinabea TJ.
olyoxometalates with Keggin-Ion Structures), Che
nistry OF Materials, see 1 .381-383 (1989).

Starting from LDH containing the corresponding nitrate or chloride, some polyoxometallate intercalates Zn / Al- and Mg / AL-
The production of LDH material is described by Waltermann in US Pat.
54,244. However, in our hands, the product formed according to the conditions mentioned was amorphous on X-ray judging from the lack of clear Bragg reflection. Nonetheless, these polyoxometallate-inserted amorphous materials showed increased SOx uptake compared to their precursors, similar to the partially inserted LDH described previously.

According to certain preferred embodiments discussed in the present invention, Fe (ox) ₃
^{When 3-} , FeO ₄ ^2- , V ₁₀ O ₂₈ ^6- , W ₇ O ₂₄ ^6- , and Mo ₇ O ₂₄ ^6- are inserted, the hydrotalcite-like Mg ₃ AL-LDH is more than ordinary hydrotalcite. It gave better SOx absorption (Table 2).

In these cases, oxides of iron, vanadium, tungsten, and molybdenum act as SO ₂ oxidation catalysts. Similarly Zn ₂
Al-LDH showed the same as polyoxoanion is inserted (Table _{2) Zn 2 Al-NO 3} LDH precursor to increased SOx uptake compared.

Furthermore, as with the amorphous LDH-POM reaction product, POM
Mg ₃ Al and Zn ₂ produced by partially inserting anions
Al-LDH absorber also produced a suitable SOx absorber. All these substances are their precursors LDH, [Zn ₂ Al (OH) ₆ ] (NO
₃₎ · xH showed ₂ O or _{[Mg 3 Al (OH) 16} ] (OH) · xH 2 O SOx uptake increased compared to. For example, V ₁₀ O ₂₈ ^6- inserted crystalline Zn ₂ Al-LDH is produced under the conditions previously described in the present invention.
It showed a weight gain of 23.6% at 700 ° C. The corresponding amorphous material showed a 20% weight gain at this temperature. Zn ₂ Al-LDH
The crystalline absorbent formed by inserting α [SiV ₃ W ₉ O ₄₀ ] ^7- into SOx showed a weight increase of 5.78% at 700 ° C relative to SOx, while β- [SiV ₃ The W ₉ O ₄₀ ] ^7- partially intercalated absorbent showed a weight gain of 5.12% under the same conditions.

In another embodiment, Mg ₃ Al-LDH in which V ₁₀ O ₂₈ ^6- is inserted, [Mg
₆ Al ₂ (OH) ₁₆ ] (V ₁₀ O ₂₈ ) _1/3 · xH ₂ O was exposed to air containing 0.5 vol% SO ₂ in the temperature range of 500-800 ° C. As is clear from the weight increase measurement (Fig. 2), better SOx absorption was observed in the temperature range of 600 to 800 ° C.

As one embodiment, it was examined whether or not these used LDH absorbents can be regenerated for recycling. The spent Zn ₂ Al-LDH absorbers after exposure to SO ₂ at 700 ° C were incorporated with SOx [and / or H] as evidenced by their weight loss upon further heat treatment (Table 3). ₂ SO ₄ ].

For example, Zn / Al-LDH containing SiV ₃ W ₉ O ₄₀ ^7- polyoxometalate anions showed a weight increase of 5.78% at 700 ° C. in the presence of air containing 0.5% by volume of SO _2. In air flow at 900 ° C
In the absence of SO ₂ , this spent sorbent (in sulfate form) released all of its bound SOx, regained its original weight prior to SO ₂ treatment and re-formed the oxidant sorbent. .
This reformed absorbent had the same activity for reaction with SOx as the original absorbent (Figure 3). Thus, the SOx sorbent can be recycled if desired by proper control of the absorption / desorption temperature.

The SOx uptake as a function of time is given in FIG. 4 for some of the Mg ₃ Al absorbers discussed in this invention. M
The absorbents disclosed in the present invention compared to conventional basic absorbents such as gO and Mg (OH) ₂ show excellent initial and total absorption, especially when the interlayer ions of the layered double hydroxide structure include iron. Shows SOx absorption. In certain reaction conditions, i.e., at 700 ° C. in a gas stream containing 0.5% of SO ₂ and air, for example, MgO 10.2
It has been found that Mg (OH) ₂ exhibits a conversion of 14.0% in 1 hour, subject to a conversion of MgO point of% to MgSO ₄ . When iron was blended so that MgO: Fe ₂ O ₃ = 3: 1 as Fe ₂ O ₃ to MgO by mixing, the conversion increased to 36.5%. Mg (OH))
_A similar mixing experiment [Mg (OH) ₂ : Fe (OH) ₃ = 3:
On the contrary, in 1], the uptake was decreased (conversion rate of 9%). Under the same set of reaction conditions, Mg ₃ containing FeO ₄ ²⁻ and Fe (ox) ₃ ³⁻ anions as disclosed in the present invention.
Al-LDH (Mg: Al = 3: 1) is 85.2% and 67.8% Mg, respectively.
The conversion rate from point to MgSO ₄ is shown. These values are far superior to MgO or Mg (OH) _{2 with} or without iron. V ₁₀ O ₂₈ ^6- , HVO ₄ ^2- , SiV ₃ W ₉ O ₄₀ ^7- , and BVW ₁₀ O
LDH with vanadium inserted containing anions such as ₄₀ ^7-
The absorbent also showed better SOx uptake values (Table 2 and Figure 4). Furthermore, these polyoxometamate anion-inserted LDH absorbers have better initial SO than MgO or Mg (OH) _2.
x showed uptake.

The previously mentioned European patent application EP-A 278 535 describes SO generated during the refining process, especially when processing high sulfur feeds.
For the purpose of binding x, a hydrocarbon pyrolysis catalyst composition containing an anionic clay having an LDH structure as a component is described. The LDH component contains rare earth metal ions (for example, L
a ³⁺ , Ce ³⁺ ), noble metals (eg Pt, Rh) and transition metal ions (Cu ²⁺ , Fe ³⁺ , Mn ²⁺ , Cr ³⁺ ) known SO ₂
Many of the oxidation catalysts are included. Rare earth metal and noble metal catalysts are preferred over transition metal catalysts, one because of their greater reactivity. It is known to those skilled in the art that transition metals, especially iron, are undesirable components in petroleum pyrolysis catalysts because they promote coke formation. However, iron is an economically attractive SO ₂ oxidation catalyst for applications where coke formation, which reduces SOx from flue gases of coal-fired power plants, is not a problem.

As a preferred invention described here, SO ₂ by an LDH substance is used.
It is disclosed that the effect of transition metal ions (especially iron and vanadium) on promoting the uptake of H2O3 depends in part on the transition metal composition. Introducing a transition metal between the layers of the LDH structure gives an order of magnitude increase in SOx absorption. This teaching is illustrated by the results given in Table 2 for SO ₂ uptake at 700 ° C. by Mg ₃ Al and Zn ₂ Al-LDH compositions with different interlayer anions consisting of SO ₂ oxidation catalysts. In the present invention, when the oxidation catalyst iron is introduced between the layers of the LDH structure in the form of FeO ₄ ^2- , for example,
The SOx uptake capacity of hydrotalcite can be measured from Mg point to MgS
It was shown to increase from 12.5% conversion to O ₄ to 85.2% conversion. This is an improvement of about 72.7% when using Mg point for SO uptake. Furthermore, the introduction of vanadium-containing polyoxometalates anions similar Fe (ox) ₃ ^3- other iron containing anions such as, SOx uptake capabilities of the LDH sorbent was significantly improved. Therefore, iron and vanadium are preferred as SO ₂ oxidation catalysts for the absorbents disclosed in this invention prepared for use in flue gas desulfurization processes.

Most of the absorbents disclosed in this invention are Mg as M ^II cations.
Contains ²⁺ . Alternatively, Ca ²⁺ -containing LDH such as hydrocalumite and its derivatives could also be used as an absorbent to eliminate SOx after the polyoxometallate has been inserted. These substances are CaO
Judging from the reactivity of the compounds with SO ₂ and SO ₃ (see background of the invention above), they should have similar or excellent SOx absorption.

Metal-containing LDHs useful as precursors for making the compositions disclosed in this invention can be synthesized from inexpensive starting materials. For example, hydrotalcite-like substance is Mg
O and alumina (Al ₂ O ₃ ) can be produced as starting materials, and the hydrocalumite-like substance can be produced from CaO and alumina. Both CaO and MgO can be obtained by calcining natural minerals such as calcite (CaCO ₃ ) and magnesite (MgCO ₃ ). Some of these layered double hydroxides, such as hydrotalcite, are commercially available and some are naturally occurring. Furthermore, their synthetic methods are also known in the art.

The SO ₂ oxidation catalyst can be introduced between the layers of LDH using a variety of different metal-containing anions other than those disclosed in this invention. These include inter ^{_{alia, V (CO) 6 -,}} Fe 2 (C
O) ₈ ²⁻ , Fe (CO) ₄ ²⁻ , Fe ₃ (CO) ₁₁ ²⁻ , Cr (CO) ₅ ^2−, or other metal carbonyl, or VCl ₄ ⁻ , CrCl ₆ ³⁻ , W ₂ C
metal halides such as l ₉ ³⁻ , FeCl ₆ ³⁻ , or Mn (C
N) ₆ ⁴⁻ , W (CM) ₈ ⁴⁻ , Fe (CN) ₆ ^{3− and} other metal nitriles, or metal oxalates, Fe (ox) ₂ ²⁻ , WO ₂ (o
x) ⁻ , Cr (ox) ₃ ³⁻ , or transition metal complex chelates such as metal acetylacetonate.

It is known to those skilled in the art that some of the transition metals, especially iron, can oxidize NO to NO ₂ . Thus, the transition metal-containing LDH absorbents described in the present invention, especially iron-containing absorbents, can be used to remove NOx components from flue gas streams and other gas streams. In the gas stream, the calcined LDH will react with NOx components to form solid nitrates.

These absorbents can be used, for example, in the form of particles of suitable size and shape. Such particles can be formed by conventional methods such as spray drying, pill formation, tablet formation, bead formation and the like.

For coal fired boilers, the absorbent of the present invention may be used with or separately from the coal in the combustion zone (eg, boiler, temperature 700
It may be added when the combustion is carried out to ~ 1000 ° C). The sorbent can then exit the combustion zone with the coal ash and be recovered from the backhouse. This method will, in turn, provide sufficient contact time for the absorbent to react with SOx from the flue gas stream. Thus, the flue gas exiting the combustion zone / contact zone system has a reduced amount of sulfur oxide compared to when treated without the absorbent of the present invention. If desired, the reacted sorbent can be separated from the ash by screening, density separation, or other well known solid separation methods (especially sorbents containing Zn and Al, for regeneration). Furthermore, the used absorbent can be safely disposed of without causing any serious environmental pollution. Because
This is because the SOx in the used absorbent is in a thermally stable sulfuric acid form. Further, the absorbents described herein could be used in other ways, such as hydrocarbon pyrolysis, where SOx from the gas stream needs to be reduced.

The following examples will serve to illustrate certain aspects of the invention disclosed herein. However, these examples should not be considered as limiting the scope of the invention. This is because many changes can be made without departing from the subject matter of the invention.

Example 1 In this example describes the preparation of hydrotalcite-like Mg ₃ Al-LDH.

12.8g of _{Mg (NO 3) 2 · 6H} 2 O and 9.4g of _{Al (NO 3) 3 · 9H} 2
A solution of O in 100 ml of deionized water was added to 14 ml of 50% Na
OH and 5 g Na ₂ CO ₃ (anhydrous) were added to a solution in 200 ml distilled water. The addition was done very slowly over 90 minutes with vigorous stirring. Following the addition, the resulting thick slurry was heated at 65 ± 5 ° C. for 18 hours with good stirring. The mixture was then cooled to room temperature and the precipitate separated by centrifugation. The solid was washed several times with deionized water until the wash was salt-free and dried in air. The X-ray diffraction pattern of the dried solid was found to correspond to hydrotalcite, and the basal plane spacing was found to be 7.78Å. Mg by chemical analysis
The / Al ratio was shown to be 3.2, and [Mg ₃ Al (OH) ₈ ] (C
It was very close to the value expected for hydrotalcite with the ideal unit of O ₃ ) _0.5 · xH ₂ O.

By varying the amounts of Mg ²⁺ and Al ³⁺ salts used, hydrotalcite-like materials with different Mg / Al ratios can be produced.

Example 2 In this example, [Zn ₂ Al (OH) ₆ ] X · zH ₂ O (X = N
O ₃ , Cl) production is described.

All operations were performed in N ₂ atmosphere and solvents were
N ₂ was about 2 hours to advance boiled in.

In 200ml of _{0.1M Al (MO 3) 3 ·} 9H 2 O solution was added NaOH solution 1.0M until the pH of the solution is 7. The white slurry was stirred for 1 hour and 200 ml of 0.3M Zn (NO ₃ ) ₂ solution was added dropwise. The pH of the mixture is adjusted by adding NaOH during addition.
Maintained at 6.0. The resulting slurry is placed in a nitrogen atmosphere for 24 hours.
It boiled for an hour. (Boiling of this suspension for one week produced a highly crystalline product). Product [Zn ₂ Al (OH) ₆ ] N
O ₃ · zH ₂ O was washed several times with water using centrifugation and dried in air. The powder X-ray diffraction pattern of the dried solid is 7.7Å
Corresponding to LDH structures with basal plane spacing values of. A Cl ^- derivative [Zn ₂ Al (OH) ₆ ] Cl.zH ₂ O was prepared using the same method.
Can be prepared using AlCl ₃ and MgCl ₂ .

By varying the amounts of Zn ²⁺ and Al ³⁺ salts used, hydrotalcite-like materials with different Zn / Al ratios can be produced.

Example 3 In this example, the general formula [Zn ₂ Al (OH) ₆ ] (POMn ⁻ ) _{1 / n} · xH ₂ O, where POM is a polyoxometallate anion with a charge of n, is used. A general method for producing a polyoxometanote-inserted Zn / Al-hydrotalcite-like substance having the same is described.

A boiling solution containing about 5 milliequivalent (mequiv.) Portion of Zn ₂ Al-X (X = NO ₃ , Cl) LDH prepared in Example 2 was added to a stirred aqueous solution containing about 7.5 milliequivalent polyoxometalate anion. Dropped. The pH of the resulting slurry after the addition is complete
Was adjusted to about 6 by adding diluted HNO ₃ acid. The slurry was stirred for about 1 hour, the solid product was separated and washed thoroughly with water using centrifugation. The powder X-ray diffraction pattern of the dried solid corresponded to the hydrotalcite-like layered structure having polyoxometallate anions contained between the layers. The basal plane spacing is given in Table 1. Zn ₂ Al by chemical analysis
It was confirmed to have a structure of (OH) ₆ [POMn ⁻ ] _{1 / n} · yH ₂ O (in the formula, POM represents a polyoxometallate having a charge of n).

Low-charge anions such as [PW ₁₂ O ₄₀ ] ^3- and [SiW ₁₂ O ₄₀ ] ^4- do not show ion exchange, whereas intermediate anions show partial insertion [eg [PCuW ₁₁ O ₃₉ (HO)]
^5- ]. Furthermore, polyoxometallate anions having a β-Keggin type structure such as β- [SiV ₃ W ₃ O ₄₀ ] ^7- , undergo only partial insertion. US Patent No. by Walter Mann
[Zn ₂ Al (OH) ₆ ] NO ₃ disclosed in 4,454,244
· ZH In the manufacture of polyoxometalate inserted Zn / Al-LDH materials from ₂ O, has occurred the amorphous substance in X-ray.

Example 4 In this example, the formula from hydrotalcite, [Mg ₆ Al ₂
(OH) ₁₆ ] OH · xH ₂ O Describes the production of Mg / Al-LDH of LDH.

A sample of hydrotalcite prepared according to Example 1 was calcined for 3 hours at 500 ° C. A 5 g portion of this sample was ground and suspended in 200 ml of degassed hot (65 ° C.) deionized water to form a white slurry. The resulting slurry was then vigorously stirred in a nitrogen atmosphere at 65 ° C. for 1 hour to form the hydroxide derivative [Mg ₃ Al (OH) ₈ ] OH.xH ₂ O. The resulting slurry was cooled to room temperature and adjusted to a volume of 250 ml with deionized water. The powder X-ray diffraction pattern of the dry solid corresponded to the hydrotalcite structure. The basal plane spacing was found to be 7.76Å.

Example 5 This example describes the general method used to prepare polyoxometallate intercalated Mg / Al-LDH.

About 25 Mg ₃ Al-LDH-OH slurry prepared according to Example 4.
A solution containing the mM portion was added to the polyoxometallate anion 40.
It was added dropwise to a stirred aqueous solution containing mM in a nitrogen atmosphere. The resulting slurry was stirred at ambient temperature for about 18 hours, the solid product was separated and washed thoroughly with water using centrifugation. The powder X-ray diffraction pattern of the dried solid corresponded to the hydrotalcite-like structure having the polyoxometallate inserted therein. The basal plane spacing is given in Table 1.

Example 6 SOx uptake by various LDH was carried out using Cahn TG
Determined by thermogravimetric analysis using a -121 thermogravimetric analyzer.

About 50 mg of sorbent was placed on the quartz pan of a thermogravimetric analysis balance. Subsequent processing of the samples was done in a three-step process.

Step 1: Equilibrate the sample at 25 ° C for 15 minutes in a stream of air (200 ml / min) as the carrier gas, then calcining temperature, typically
Heated slowly to 700 ° C (5 ° C / min). 1 more sample
Maintained at this temperature for hours.

Stage 2: SO ₂ gas (0.5%) was then introduced into the carrier gas at that temperature and the weight was checked for 1 hour. For more reactive absorbents, especially those containing iron, SOx
A rapid initial weight gain of was observed. The weight gain corresponded to the amount of SO ₃ absorbed by the calcined sample (Table 2). See Figure 1 for a typical TGA plot.

Step 3: Stopping SO _{2 in the} carrier gas and checking the sample weight at the reaction temperature for another hour. This stage is
It was carried out to determine the thermal stability of the metal sulfate product formed after reaction with SOx. All the samples with Mg and Al in the layers showed little or no weight loss, while most samples with Zn in the layers showed considerable weight loss (Table 3).

Example 7 Hydrotalcite-like materials with inserted metal polyoxoanions similar to those of Examples 3 and 5 were tested for SOx absorption at 700 ° C according to the procedure of Example 6. The results are shown in Table 2. LDH containing Mg and Al showed better SOx absorption than those containing Zn and Al. The preferred polyoxoanion was FeO ₄ ²⁻ , which showed greater than 85% conversion of Mg point to MgSO ₄ upon exposure to SO ₂ . But,
The polyoxoanion containing V, W and Mo also showed increased SOx absorption compared to hydrotalcite without them. The corresponding Zn ₂ Al-LDH formed a metastable product with SOx compared to the polyoxometallate intercalated Mg ₃ Al-LDH. With further calcined in the absence of SO ₂ (Example 6, step 3), most of these spent sorbent showed a weight loss (Table 3). Therefore, some of these spent absorbents can be regenerated for further use (see Example 9).

Example 8 A hydrotalcite-like material having V ₁₀ O ₂₈ ^-6 inserted according to Example 4 was subjected to various calcination temperatures according to the procedure of Example 6.
Tested for SOx absorption. Results are shown in FIG. The preferred calcination temperature is in the range of 550 ~ 800 ℃, 32% ~ 52%
A conversion rate from Mg point to MgSO ₄ was observed. At very high temperatures (> 900 ° C) uptake was very low.

Example 9 Zn ₂ A intercalated with polyoxometallate according to Example 5
Several l-LDH, after exposure to SO _2, regenerated and tested for SO ₂ resorption using the following procedure.

About 50 mg of SiV ₃ W ₉ O ₄₀ ^8- inserted Zn ₂ Al-LDH was tested for SOx uptake using steps 1 and 2 of the procedure shown in Example 7 and the sample was further processed as follows.

Step 3: Turn off SO ₂ in the carrier gas, heat the absorbent to 800 ° C (5 ° C / min) and bring the temperature to this value for another 30
Hold for minutes and cool to 700 ° C (-5 ° C / min).

Step 4: SO ₂ gas (0.5%) was reintroduced into the carrier gas at 700 ° C. for 1 hour.

Step 5: Stop adding SO ₂ and bring the reaction temperature to 700 ° C for another 1
Held for hours.

A 5.6% weight gain was observed when SO ₂ was first introduced during stage 2 (FIG. 3). Almost all of this weight gain (90
%) Was shown to have been released from the spent sorbent and absorbed SOx in step 3 at higher temperatures. The SO ₃ uptake by this regenerated absorbent was 6.4 in step 4.
%, Indicating that the regenerated material was as good as the original absorbent.

The above description is merely illustrative of the present invention, which is limited only by the claims.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/81 B01J 20/04 A 7202-4G 20/06 A 7202-4G 20/08 A 7202- 4G C01B 17/76 A (72) Inventor Polanski, Christine, A. USA 48847 South Jerome Road, Ithaca, Michigan 975 (56) References US Pat. No. 4866019 (US, A)

Claims (24)

[Claims] 1. A method for removing SOx components from a flue gas stream containing oxygen, sulfur dioxide, and sulfur trioxide by combustion of coal from a coal-fired boiler in a boiler for supplying the flue gas stream. Burning coal at 40 ° C., and heating the absorbent composition and the gas flow from 400 ° C. to 1000 ° C.
Contacting between, the absorbent before heating,
Layered double hydroxide structure of the following formula: [M _1-X ^II M _X ^III (OH) ₂ ] (An ⁻ ) _{X / n} · yH ₂ O (wherein M ^II is a divalent metal cation M ^III is a cation capable of forming a metal oxide, reacting with SO ₂ to form a metal sulfite, and reacting with SO ₃ to form a metal sulfate, II A, II A trivalent metal cation selected from the group consisting of B and Group III A metals, where A is
The layered double hydroxide structure provides the oxidation of sulfur dioxide to sulfur trioxide in an amount sufficient to promote the oxidation of sulfur dioxide to sulfur trioxide under combustion conditions in coal-fired boilers. An anion of an interlayer n of charge n ⁻ containing at least one metal atom selected from the group consisting of group metals and transition metals, where y is the number of water molecules). A method of removing SOx from a flue gas stream from a coal-fired boiler. 2. The method of claim 1, wherein M ^II is at least partially an alkaline earth metal cation. 3. The method of claim 2, wherein M ^II is an alkali metal cation selected at least in part from the group consisting of magnesium and potassium cations. 4. An M ^II metal cation at least in part,
The method of claim 1 selected from the group consisting of transition metal cations. 5. The method of claim 4, wherein the transition metal cation is a zinc cation. 6. The method of claim 1, wherein M ^III is selected in part from the group consisting of Group III A metal cations. 7. The method of claim 6, wherein the metal cation is an aluminum cation. 8. The method according to claim 1, wherein the anion A is a polyoxometallate anion. 9. The method according to claim 8, wherein the polyoxometallate anion is isopolyoxometallate containing one metal atom and oxygen. 10. Anion is CrO ₄ ²⁻ , FeO ₄ ²⁻ , HVO ₄ ²⁻ ,
The method according to claim 9, which is selected from MoO ₄ ^2- , V ₁₀ O ₂₈ ^6- , Mo ₇ O ₂₄ ^6- , W ₇ O ₂₄ ^6- , and mixtures thereof. 11. The method of claim 8 wherein the polyoxometallate anion is a heteropolyoxometallate containing more than one metal atom and oxygen. 12. The method according to claim 11, wherein the heteropolyoxometallate anion is an anion having a Keggin structure. 13. The Keggin anion is H ₂ W ₁₂ O ₄₀ ^6- , SiV ₃ W ₉
13. The method of claim 12 selected from the group consisting of O ₄₀ ^7- , BVW ₁₀ O ₄₀ ^7- , and mixtures thereof. 14. A heteropolyoxometallate anion having the formulas [XM ₁₁ O ₃₉ ] n ⁻ and [XM ₉ O ₃₄ ] n ⁻ (wherein one of X and M is a metal of the main group and 12. The method of claim 11, wherein the anion has a vacancy (defect) Keggin structure having a metal selected from the group consisting of transition group metals. 15. The vacancy (defect) Keggin anion is BCoW.
15. The method of claim 14 selected from the group consisting of ₁₁ O ₃₉ ^9- , SiW ₁₁ O ₃₉ ^9- , and mixtures thereof. 16. The method of claim 11, wherein the heteropolyoxometallate anion is an anion selected from strong anions consisting of fused Keggin-type structures. 17. The method of claim 16, wherein the fused Keggin anion is BCoW ₁₂ O ₄₂ ^8- . 18. The method according to claim 1, wherein the anion A is a metal oxalate anion. 19. A metal oxalate (ox) anion Fe (o)
The method according to claim 18, characterized in that it is an iron oxalate such as x) ₃ ^3- . 20. The method of claim 1 wherein M ^II is Zn and is regenerated by heating to remove bound SOx and separating the absorbent for reuse. 21. The anion A metal is iron, vanadium,
The method of claim 1 selected from the group consisting of molybdenum and tungsten. 22. The method according to claim 1, wherein the anion A is FeO ₄ ²⁻ . 23. The method according to claim 1, wherein the anion A is Fe (ox) ₃ ³⁻ . 24. Sulfur oxide at the highest temperature of a coal-fired boiler, M ^II and M ^III being heated to the second highest temperature to remove said sulfur oxide and for reuse. The method of claim 1, wherein the heated absorbent composition is regenerated.